The present invention relates to a method and apparatus for determining channel degradation information in which a known data sequence is compared to a transmitted version of the known data sequence to provide such information.
Signals carried over telecommunications links can undergo considerable transformations, such as digitisation, encryption and modulation. They can also be distorted due to the effects of lossy compression and transmission errors.
Objective processes for the purpose of measuring the quality of a signal are currently under development and are of application in equipment development, equipment testing, and evaluation of system performance.
A number of patents and applications relate to this field, for example, European Patent 0647375, granted on 14 Oct. 1998. In this invention two initially identical copies of a test signal are used. The first copy is transmitted over the communications system under test. The resulting signal, which may have been degraded, is compared with the reference copy to identify audible errors in the degraded signal. These audible errors are assessed to determine their perceptual significance—that is, errors that are considered significant by human listeners are given greater weight than those that are not considered so significant. In particular inaudible errors are perceptually irrelevant and need not be assessed.
The automated system provides an output comparable to subjective quality measures originally devised for use by human subjects. More specifically, it generates two values, YLE and YLQ, equivalent to the “Mean Opinion Scores” (MOS) for “listening effort” and “listening quality”, which would be given by a panel of human listeners when listening to the same signal. The use of an automated system allows for more consistent assessment than human assessors could achieve, and also allows the use of compressed and simplified test sequences, which give spurious results when used with human assessors because such sequences do not convey intelligible content.
In the patent specification referred to above, an auditory transform of each signal is taken, to emulate the response of the human auditory system (ear and brain) to sound. The degraded signal is then compared with the reference signal in the perceptual domain, in which the subjective quality that would be perceived by a listener using the network is determined from parameters extracted from the transforms.
Such automated systems require a known (reference) signal to be played through a distorting system (the communications network or other system under test) to derive a degraded signal, which is compared with an undistorted version of the reference signal. Such systems are known as “intrusive” measurement systems, because whilst the test is carried out the channel under test cannot, in general, carry live traffic.
The present invention has applications in, but is not limited to, measuring the signal degradation caused by transmission over a digital radio channel. European patent application EP 01306950.5 describes how the perceived transmission quality of a digital radio channel can be evaluated using channel degradation information in the form of error patterns to generate a reference and degraded signal pair for use with an intrusive measurement system. Error patterns store the difference between a reference digital sequence and a degraded version received after transmission over an error-prone channel. In patent application EP 01306950.5, a novel means of generating a known test sequence for the purposes of generating error patterns is presented.
When testing the performance of a public land mobile radio network (PLMN), it is desirable to locate the apparatus used to perform the signal quality measurement apparatus in the network rather than the mobile station. Network based apparatus can be utilized more efficiently than mobile station based apparatus, by dynamically allocating it to active channels. Apparatus located in a mobile station will only be utilized when the mobile station is active, which in most cases represents a small fraction of time. This requirement to perform the quality measurement in the network presents a problem when measuring the performance of the downlink (network to mobile station) channel, because the degraded digital sequence used to form an error pattern is received at the mobile station.
European Patent application No. 00304497.1 describes a method and apparatus for measuring the performance of a communications channel while in normal use by exploiting periods of spare capacity. The invention described therein can be implemented according to the arrangements described below. The inventors of said patent solve the problem of downlink measurement by making provision to send an error protected version of the degraded digital sequence received at the mobile station back to the network, where it can be compared with a copy of the original to produce an error pattern.
A problem with transmitting an error corrected version of the degraded digital sequence is that the forward error correction process increases the amount of data that must be sent. If the transmission of the degraded signal is to be robust over a wide range of radio conditions, a powerful forward error correction code must be used, causing a substantial increase in the amount of data to be sent. For example, a rate 1/6 convolutional code will increase the amount of data to be transmitted by more than a factor of six. In many systems this increase in data will exceed the capacity of the transmission channel, especially if transmission is limited to periods of spare capacity, and the fraction of time for which signal quality measurements can be reported will be reduced. There will also be a need for a large buffer in which to store the degraded data sequences awaiting transmission.
The present invention provides a means of generating channel degradation information on the network side of a digital transmission channel that is derived from the error performance of the downlink channel. The invention alleviates the problem of buffering and transmitting large amounts of data over the uplink channel by sending a statistical representation characterising the errors in the degraded data sequence. This information is used to generate a representation of the channel degradation information with characteristics similar to the channel degradation information generated directly from the degraded data sequence. The statistical representation can be represented in relatively few bits, and can therefore be protected by powerful forward error correction codes without exceeding the transmission capacity of the channel. If transmission is limited to periods of spare capacity, the smaller amount of information also reduces the memory requirements of the buffer.
In some PLMN systems, provision is made to characterise the error performance in one direction, and return this information over the reverse channel. An example of this technique is the RXQUAL value that is calculated in GSM receivers (see GSM 05.08). However, the present invention is distinct from RXQUAL type measurements, because errors are accurately identified by comparing the received sequence with a local copy of the original sequence; RXQUAL is based on an estimate of the error rate calculated over 480 ms, and has been shown in the literature to provide an unreliable predictor of speech quality (Radio link parameter based speech quality index-SQI Karlsson, A.; Heikkila, G.; Minde, T. B.; Nordlund, M.; Timus, B.; Wiren, N. and Electronics, Circuits and Systems, 1999. Proceedings of ICECS '99. The 6th IEEE International Conference on , Volume: 3, 1999 Page(s): 1569–1572 vol.3.)